Under encoding standards of moving-image data such as H.264, MPEG2 and the like, a macro block which satisfies a predetermined condition can be encoded as a skipped macro block (which will hereinafter be referred to as a “skipped MB”). The predetermined condition is that all of quantization values of a macro block take on zero when a predictive motion vector is used. In a case where a macro block is encoded as a skipped MB, encoded data of the macro block is not generated. An image decoding device uses data in a source of reference of the predictive motion vector in decoding the skipped MB. The use of a skipped MB can improve efficiency in encoding moving-image data.
However, in some cases, in order to decrease power consumption or reduce delay, encoded moving-image data is decoded with a method other than well-known methods. In such cases, if moving-image data is encoded with the use of a skipped MB, an error may probably be caused when the encoded moving-image data is decoded. In view of this, Patent Documents 1 and 2 describes techniques of inhibiting encoding of a specific macro block as a skipped MB.
Patent Document 1 describes an image encoding device which divides a picture into a plurality of parallel processed areas, and encodes the plurality of parallel processed areas in parallel with the use of a plurality of encoding elements. Each of the encoding elements deters generation of a skipped MB in a leading macro block of each of the parallel processed areas. As a result of this, when one encoding element encodes one parallel processing area, it can prevent from referring to the quantization parameter in the other parallel processing area without a need to divide a picture into a plurality of slices.
Patent Document 2 describes techniques of dividing a picture into more than one slice to encode the picture. When the first macro block or the last macro block of each of the slices is encoded as a skipped MB, the first macro block or the last macro block which is encoded as a skipped MB is forcefully changed in type into a non-skipped MB.
Patent Document 1: Japanese Patent Application Publication No. 2009-239565
Patent Document 2: Japanese Patent Application Publication No. 2000-333180
The image encoding device according to Patent Document 1 divides a picture into a plurality of parallel processed areas and encodes the plurality of parallel processed areas in parallel, to thereby enhance processing performance with low power consumption. By using a structure of the image encoding device according to Patent Document 1, it is possible to shorten a time required for encoding of a picture. However, the image encoding device according to Patent Document 1 needs to include a plurality of encoding elements, which complicates the structure.